Sintered connecting rod for an internal combustion engine

ABSTRACT

A sintered connecting rod for an internal combustion engine comprises a connecting rod eye ( 1 ) for receiving a piston pin, a connecting rod big end ( 3 ) for holding the crank pin and a shaft ( 2 ) made of two diverging longitudinal bridges ( 4 ) which are optionally stiffened by a connecting bridge ( 5 ) and whose mutually facing bridge flanks ( 11 ) are connected in the zone of the connecting rod big end ( 3 ) by a central concave rounding ( 9 ). In order to ensure advantageous loading conditions it is proposed that the longitudinal bridges ( 4 ) of the shaft ( 2 ) diverge from the connecting rod big end ( 3 ) to the connecting rod eye ( 1 ).

TECHNICAL FIELD

[0001] The invention relates to a sintered connecting rod for aninternal combustion engine with a connecting rod eye for receiving apiston pin, with a connecting rod big end for holding the crank pin andwith a shaft made of two diverging longitudinal bridges which areoptionally stiffened by a connecting bridge and whose mutually facingbridge flanks are connected in the zone of the connecting rod big end bya central concave rounding.

RELATED ART

[0002] In order to allow utilizing advantages caused by sinteringconcerning the adherence to weight tolerances on the one hand andconcerning simpler production on the other hand it has already beenproposed that connecting rods for internal combustion engines byproduced by sintering hot-pressed powder blanks. The shafts of saidconnecting rods are formed by two longitudinal bridges which areoptionally stiffened by a connecting bridge and which diverge from thepiston rod eye to the piston rod big end. This embodiment of connectingrods was capable of meeting the static and dynamic strengthrequirements, especially in the case of vehicle engines, this beingdespite the lower strength of the sintered connecting rods in comparisonwith forged connecting rods. Due to the larger shaft width in thetransitional region to the connecting rod big end, such sinteredconnecting rods are hardly suitable for use in short-stroke internalcombustion engines because the larger shaft width in the transitionalregion to the connecting rod big end considerably reduces the possiblepivoting angle of the connecting rod which must not touch the cylinder.Efforts to reduce the shaft width in the transitional region to theconnecting rod big end by parallel bridges and smaller transitionalradii on the outside flanks of the longitudinal bridges have led to animpermissible reduction of the security against fatigue fractures,especially in the region of the oil ducts which are provided directlyadjacent to the shafts in the connecting rod big ends in order to supplyoil for lubricating and cooling purposes to the cylinders and pistonsvia the crank shaft.

DESCRIPTION OF THE INVENTION

[0003] The invention is thus based on the object of providing a sinteredconnecting rod of the kind mentioned above in such a way that it can beused for short-stroke internal combustion engines without endangeringsufficient security against fatigue fractures.

[0004] The invention achieves this object in such a way that thelongitudinal bridges of the shaft diverge from the connecting rod bigend to the connecting rod eye.

[0005] Since as a result of this measure the shaft width decreases fromthe connecting rod eye to the connecting rod big end, it is possible toensure connecting rod dimensions, despite sufficiently largetransitional radii between the outside shaft flanks and the connectingrod big end, which allow an unlimited use in short-stroke internalcombustion engines, namely with a sufficient security against fatiguefractures. The bridges of the shaft which diverge towards the connectingrod eye produce an even distribution of stress in the transitionalregion between the connecting rod eye and the shaft, which supports theeven loading of the two longitudinal bridges of the shaft and providesadvantageous prerequisites for pressing the piston pin into theconnecting rod eye. In the region of the connecting rod big end,however, a stress concentration is obtained in the transitional regionby the converging longitudinal bridges which leads to the effect thatthe regions adjacent to the circumferential side are relieved. Thiscontributes positively to the stress state in the region of an oil duct,if any, which comes to lie in the region of this stress relief, so thatthe critical stresses in the region of the oil ducts are within apermissible magnitude. The embodiment of the sintered connecting rods inaccordance with the invention thus allows adjusting the dynamic strengthbehavior to the occurring loads in an advantageous manner and ensuring asufficient security against fatigue fractures even in critical regions.

[0006] Since the stress concentration in the transitional region betweenthe shaft and the connecting rod big end depends on the arrangement ofthe rounding between the inner bridge flanks of the longitudinalbridges, it is possible to having an influence on the fatigue fracturebehavior by way of the arrangement of said rounding. As a result, thebridge flanks and the central rounding can mutually converge by way ofconnecting roundings for the purpose of a further-reaching relief of thecircumferential regions of the piston rod big end which are adjacent tothe shaft, with the axes of said connecting roundings being situatedoutside of the shaft on the side of the opposite longitudinal bridge. Asa result of this measure it is possible to reduce the radius of thecentral rounding irrespective of the progress of the bridge flanks,which leads to the desired stress concentration. The axes of theconnecting roundings which are situated outside of the shaft and whichallow a continuous transition between the bridge flanks and the centralrounding ensure sufficiently large radii of curvature for the connectingroundings in order to ensure an advantageous stress progress within thelongitudinal bridges of the shaft. Especially advantageousconstructional conditions are obtained in this connection when the radiiof curvature of the connecting roundings correspond to at least tentimes, preferably at least twenty times, the radius of the centralrounding. The radius of the central rounding should not exceed a maximumvalue of 5 mm.

[0007] Although the angle under which the two longitudinal bridges ofthe shaft diverge depends on different constructional parameters,advantageous conditions are obtained for most applications when the twolongitudinal shafts diverge under a minimum angle of 2°.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The object of the invention is explained below in an exemplarymanner by reference to the enclosed drawings, wherein:

[0009]FIG. 1 shows a sintered connecting rod in accordance with theinvention for an internal combustion engine in a schematic, partlyvertical sectional view;

[0010]FIG. 2 shows said connecting rod in a sectional view along lineII-II in FIG. 1;

[0011]FIG. 3 shows a sintered connecting rod in accordance with theinvention in a simplified graphical representation in which regions withdifferent security against fatigue fracture are entered, and

[0012]FIG. 4 shows a section of a connecting rod in accordance with theinvention in an axially normal sectional view through the connecting rodbig end in the region of the oil duct on an enlarged scale, with regionswith different security against fatigue fractures also being shown.

EMBODIMENTS OF THE INVENTION

[0013] The illustrated connecting rod shows in the conventional manner aconnecting rod eye 1 for pressing in a piston pin, a shaft 2 and aconnecting rod big end 3 at the other end of shaft 2. Shaft 2 is formedby two longitudinal bridges 4 which diverge from the connecting rod bigend 3 towards the connecting rod eye 1 and are stiffened by a connectingbridge 5 in order to ensure the highest possible natural frequency.

[0014] In order to produce this connecting rod, a molded body made of asuitable sintering powder is hot-pressed at first before said moldedbody is provided with bores 6 for screw-type bearings and with an oilduct 7. After the sintering of the molded body, the connecting rod bigend can be divided by means of separation by breakage along apredetermined breaking surface 8 which is indicated in FIG. 1 in adot-dash line, so that the a respective cap piece of the crank pinbearing as is to be received by the connecting rod big end 3 isobtained.

[0015] As a result of the dynamic strain on connecting rods, thesecurity against fatigue fractures in respect of such connecting rods isof particular importance. In order to compensate the lower strength ofsintered connecting rods and to limit the shaft width in thetransitional region towards the connecting rod big end 3, thelongitudinal bridges 4 of shaft 2 converge against the connecting rodbig end 3. The thus resulting shifts in stresses in the occurringpressure, tensile and bending loads lead to different securities againstfatigue fractures in said regions in combination with the respectivestrength properties of said regions, as are indicated in FIGS. 3 and 4according to respective calculations. The regions of the connecting rodwithout the dots show a security against fatigue fracture which ishigher than 3.5. The regions with few dots show a security againstfatigue fracture of between 2.5 and 3.5 and the regions with more dotsshow a security against fatigue fracture of between 1.6 and 2.5. As isshown in FIG. 4, the security against fatigue fracture in the region ofthe oil duct 7 lies over the required value of 1.6. this is onlypossible because the region of the oil duct 7 can be relieved by astress concentration in the central region between the two longitudinalbridges 4. This stress concentration is achieved by a central rounding 9between the longitudinal bridges 4 which shows a comparatively smallrounding radius of 2.5 mm for example. In order to provide such arounding 9 with a small diameter between the longitudinal bridges 4without obtaining an unfavorable stress distribution in the longitudinalbridges, the rounding 9 converges continuously via connecting curvatures10 into mutually facing bridge flanks 11, with the radius of curvature rof said connecting curvatures 10 corresponding to 50 times the radius ofrounding 9 for example. These ratios of dimensions are indicated in FIG.1.

[0016] Favorable stress distributions are also obtained in the region ofthe connecting rod eye 1 as a result of the longitudinal bridges 4 whichdiverge towards said connecting rod eye 1 under an angle of 40 forexample, which stress distributions allow pressing in the piston pin.The securities against fatigue fracture lie above the required minimumvalues also in the region of the connecting rod eye 1.

1. A sintered connecting rod for an internal combustion engine with a connecting rod eye (1) for receiving a piston pin, with a connecting rod big end (3) for holding the crank pin and with a shaft (2) made of two diverging longitudinal bridges (4) which are optionally stiffened by a connecting bridge (5) and whose mutually facing bridge flanks (11) are connected in the zone of the connecting rod big end (3) by a central concave rounding (9), characterized in that the longitudinal bridges (4) of the shaft (2) diverge from the connecting rod big end (3) to the connecting rod eye (1).
 2. A connecting rod as claimed in claim 1, characterized in that bridge flanks (11) and the central rounding (9) converge into each other via connecting roundings (10) whose axes are each situated outside of the shaft (2) on the side of the opposite longitudinal bridge (4).
 3. A connecting rod as claimed in claim 1 or 2, characterized in that the radii of curvature (r) of the connecting roundings (10) correspond at least to 10 times, preferably at least twenty times the radius of the central rounding (9).
 4. A connecting rod as claimed in one of the claims 1 to 3, characterized in that the two longitudinal bridges (4) of the shaft (2) diverge under a minimum angle of 2°. 